1. Field of the Invention
This invention relates generally to an anode assembly for rotating x-ray anode tubes, and in particular to a composite substrate comprising a graphite member.
2. Description of the Prior Art
The longevity and efficiency of rotating x-ray anode tubes can be increased by using anode discs capable of high heat storing and high heat dissipating properties. Graphite possesses an exceptionally high thermal capacity when compared to molybdenum and tungsten, other materials used for making the substrate of the disc. At 1000.degree. C., the ratio of thermal capacity, in relative units, and in the order mentioned heretofore, is 48:7.4 and 48:4.1. The ratio of emissivity at 1000.degree. C. is 0.85:0.15 in both instances. However, the difficulty in using graphite as a substrate material is the problem of how to join the anode target to the graphite substrate.
Prior art anode assemblies embodying a graphite substrate suggest the use of zirconium or hafnium as a suitable material for joining the anode target to the graphite substrate. However, both of these materials are carbide formers and present the problem of how to minimize the amount of carbide formed during the joining operation, as well as during the desired working lifetime of the anode assembly, usually 10,000 x-ray exposures, minimum. The working lifetime subjects the anode assembly temperature to being cycled to reasonably high levels, the order of 1200.degree. C., and, therefore, continued carbide formation is a distinct possibility. The mechanical properties of a carbide layer formed in such an anode assembly may preclude the use of such an anode assembly in rotating x-ray anode tubes subjected to large amplitude thermal cycling.
An object of this invention is to provde a new and improved anode disc embodying a substrate comprising graphite for use in rotating x-ray anode tubes.
Another object of this invention is to provide a new and improved substrate for an anode disc which is a composite assembly including a graphite member.
Other objects of this invention will, in part, be obvious and will, in part, appear hereinafter.
In accordance with the teachings of this invention, there is provided a composite substrate suitable for use in rotating x-ray anode tubes. The composite substrate includes a first member to which an anode target may be affixed. This first member may be of tungsten, a tungsten alloy, molybdenum or a molybdenum alloy.
Affixed to the first member is a second member made of graphite. A layer of metal which includes a non-carbide forming material joins the two members together to form the composite substrate. Suitable non-carbide forming materials for use in the metal layer are platinum, palladium, rhodium, osmium and ruthenium. When the metal is platinum, up to 1% by weight by chromium may be included therein even though it is known to aid the formation of carbides.